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Show 396 GEOGRAPHICAL DISTRIBUTION. CuAP. XII. a certain degree on the lapse of ~ime, and as during changes of level it is obvious :hat Islands separated by h ll W channels are more likely to have been con-sao 'd h . tinuously united within a recent perio to t e main- 1 d than islands separated by deeper channels, we can ::derstand the frequent relation between the depth of the sea and the degree of affinity of ~he ma~malian inhabitants of islands with those of a neighbouring continent,- an inexplicable relation on the view of inde-pendent acts of creation. All the foregoing remarks on the inhabitants of oceanic islands,-na1nely, the scarcity of kinds-the richness in endemic forms in particular classes or sections of classes,-the absence of whole groups, as of batrachians and of terrestrial mammals notwithstanding the presen;e of aerial ba ts,-the singular propo:tions of certain orders of plants,-herbaceous forms having been developed into trees, &c.,-seem to me to accord bet:er with the view of occasional means of transport having been largely efficient in the long course of time, than with the view of all our oceanic islands having been formerly connected by· continu~us land w~th t~e nearest continent· for on this latter view the migratwn would probably 'have been more complete ; ~nd if modification be admitted, all the forms of hfe would have been more equally modified, in accordance with the paramount importance of the relation of organism to orgall1sm. . I do not deny that there are many and grave diffi-culties in understanding how several of the inhabitants of the more remote islands, whether still retaining the same specific form or modified since their arrival, c?~1ld have reached their present homes. But the probability of n1any islands having existed as halting-places, of which not a wreck now remains, must not be over- CHAP. XII. OCEANIC ISLANDS. 397 looked. I will here give a single instance of one of the cases of difficulty. Almost all oceanic islands even the most isolated and smallest, are inhabited b; land-shells, generally by endemic species, but sometimes by species found elsewhere. Dr. Aug.· A. Gould has given several interesting cases in regard to the landshells of the islands of the Pacific. Now it is notorious that land-shells are very easily killed by salt; their eggs, at least such as I have tried, sink in sea-water and are killed by it. Yet there must be, on my view, some unknown, but highly efficient means for their transportal. Would the just-hatched young occasionally crawl on and adhere to the feet of birds roosting on the ground, and thus get transported? It occurred to me that land-shells, when hybernating and having a membranous diaphragm over the mouth of the shell might be floated in chinks of drifted timber acros~ moderately wide arms of the sea. And I found that ~everal . spe?ies did in this state withstand uninjured an Immersion m sea-water during seven days: one of these shells was the Helix pomatia, and after it had again hybe~nated I put it in sea-water for twenty days, an~ It perfectly recovered. As this species has a thiCk calcareous operculum, I removed it, and when it had formed a new membranous one, I immersed it for fourteen days in sea-water, and it recovered and crawled away : but more experiments are wanted on this head. The ~ost ~triking and important fact for us in regard to the Inhabitants of islands, is their affinity to those of the ~earest mainland, without being actually the same species. Numerous instances could be given of this fact. I will give only one that of the Galapao-os A h . ' 5 rc Ipelago, situated under the equator, between 500 and 600 miles from the shores of South America. Here |